Nope, that's not the case. If you remember, Baltar's test DID flag Boomer as a Cylon, but he falsified the results, and then sabotaged the machine that did the testing. Plus the Cylon babes are NOT human, unless that little glow in the dark spinal column is a normal part of human physiology of which I'm not aware... Or maybe it is normal, since nobody on the show is human, technically.
Mark
Right, but this only raises the question of why they should be so hard to distinguish then that only the genius Baltar could develop a test (which involved analysis on a cellular level). Given that Cylon-people have spines that can phosphoresce under certain conditions, while human-people don't, that's a pretty obvious and noticeable difference (involving, at the very least, a radically different biochemistry in/near the spinal column) and should make for a pretty simple test. For some reason either no one in BSG is able to test for this fairly distinctive biochemical property or no one has noticed it which they would if they were looking in the slightest, i.e. dissecting Cylon corpses. Which apparently they haven't been for at least a few years. Which is unbelievably unrealistic. And again, we were told very early on that the biological difference was undetectable or very nearly so. This wouldn't be true of a humanoid species whose spine could glow red.
Some contradiction is in there somewhere.
Or maybe it is normal, since nobody on the show is human, technically.
In fact, everyone is human, IMHO. First, we now know that the BSG-people and Cylon-people are the same species: they can interbreed. The only remaining question is whether the 'people' on BSG are the same as the people (humans) on Earth. You can either answer this with an obvious yes by simply observing that they all appear to be human, or you can work within the logic of the show (the 13th tribe, prophecies etc.) to deduce that they must be the same species of people as whoever lives/lived on earth - i.e. humans.
So they are all humans. They just don't realize it (or most of them don't), instead slaved to a mindset in which some people are 'Cylons' and other people are 'humans' and most everyone on the show acts as if the two are somehow different even though (except for a few contradictory things, like the glowing spine) the biological difference is nil for all intents and purposes. In a way the show is best understood as being about extreme, idiotic blind bigotry. 95% of the conflicts and conversations wouldn't need to occur if everyone on the show (not to mention the writers) weren't so dense as to miss the obvious evidence right in front of their eyes the whole time, that the skinjobs are nothing more, or less, than (cloned) humans (in most cases humans with sociopathic tendencies), and believing otherwise cannot be justified.
p.s. I've left out one other biological difference hinted at - the Super-strength sometimes shown by Cylons in certain scenes. But I claim this is just another factoid that proves my point: there's no difference between skinjobs and humans, and when a difference is shown, it is self-contradictory and spurious. After all, if the skinjobs really do have super-strength, this would be biologically detectable either in their bones or muscles or both. (Right?) They can't be both super-strong compared to humans and be biologically (nearly) indistinguishable from humans. Yet this is what BSG asks you to believe, if you take it at face value (which, of course, I don't).
The more likely explanation here is another TV one: they gave the Cylons 'super-strength' (when they feel like it) so that they could have cool scenes involving hot chicks kicking and punching people and slamming them into walls. In other words, like so much else on BSG, this aspect of the universe doesn't make intrinsic sense - it only makes TV-ratings sense.